In recent years, photographic apparatus such as cameras have spread extensively, and the occasion to use a silver halide light-sensitive material was increased.
Under the circumstances, the improvement of silver halide light-sensitive materials toward a much higher sensitivity is strongly demanded.
One of the controlling factors to the sensitivity improvement of silver halide light-sensitive materials is in silver halide grains. Various attempts have been made to develop silver halide grains having high sensitivities.
Techniques have been studied to improve the sensitivity-to-size ratio per silver halide grain, and as one of such techniques, a technique to use tabular silver halide grains is disclosed in Japanese Pat . O.P.I. Pub. Nos. 111935/1983, 111936/1983, 111937/1983, 113927/1983, 99433/1984.
When these tabular silver halide grains are compared with the so-called regular silver halide crystal grains such as octahedrons, tetradecahedrons or hexahedrons, the surface area of a silver halide grain is larger in the same volume. Accordingly, it is understood that much more sensitizing dye can be adsorbed on the surface of a silver halide grain and a much higher sensitivity can be obtained.
On the other hand, development of spectral sensitizers has been made has been in order to attain a much higher spectral sensitivity. For example, as techniques to obtain a silver halide photographic light-sensitive material improved in spectral sensitivity in a green light area, combination of two types of oxacarbocyanines is disclosed, for example, in Japanese Pat. Exam. Pub. No. 32753/1969 and Japanese Pat. O.P.I. Pub. No. 23931/1977; combination of an oxacarbocyanine and a benzimidazolocarbocyanine is disclosed, for example, in Japanese Pat. O.P.I. Pub. No. 116646/1984; and combination of an oxacarbocyanine and an oxathiacarbocyanine is disclosed, for example, in Japanese Pat. O.P.I. Pub. Nos. 42750/1985 and 167348/1988.
Further, a silver halide photographic light-sensitive material improved in spectral sensitivity in a red light area, in which two types of thiacarbocyanines are combined, is disclosed, for example, in Japanese Pat. Exam. Pub. Nos. 4933/1968, 8741/1972 and 5781/1976.
Most of these conventional techniques, however, are liable to cause desensitization when applied to a silver halide light-sensitive material comprised of multiple silver halide emulsion layers.
The reason for the occurrence of such a disadvantage which scarcely appears in a monolayered configuration is not clear, but it can be attributed to the desorption or rearrangement of adsorbed dyes resulting from the multilayered configuration.
When these sensitizing dyes are added to a tabular grain silver halide emulsion, the adsorption of the dyes to silver halide grains does not increase despite of increase in grain surface area. As a result, the sensitization is limited to a lower level than expected.
In order to eliminate such disadvantages, there have been taken various measures to increase the adsorption of the dyes, such as modification of the halide composition in a silver halide emulsion or addition of halogens to an emulsion. But the modification of an emulsion changes the emulsion's ripening conditions, thereby adverse influences are exerted on the balance of photographic properties among layers and the preservability of an emulsion. Therefore, these methods are limited in effectiveness and cannot employ the merit of tabular silver halide grains adequately.
Under the circumstances, there has been desired a tabular silver halide emulsion which is free from the above problems and high in spectral sensitivity.